686 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
prosenchyma appears as if it terminated abruptly at its inner margin ; but this appear- 
ance is only due to the destruction of the more delicate central tissue. In Plate LIII. 
figs. 21 & 22, the two tissues are seen to graduate into each other. At the extreme 
periphery of the sections the cells become smaller; some of the larger ones have a 
diameter of as much as *0025, whilst the lesser ones are not more than *0012. This 
thick prosenchymatous cortical layer has every appearance of having been hard and 
woody, as is the case with the corresponding tissue in many of the living Adiantums. 
In the transverse section of a smaller stem (Plate LIII. fig. 21), the only change 
observable is the consolidation of the four vascular bundles into a common bundle, and 
in the much smaller size of its component vessels. Its transverse section still retains 
its crucial form. In Plate LIII. fig. 22, representing a yet smaller stem, the transverse 
section of the bundle has become distinctly trifid. In some specimens this tripartite 
feature is yet more striking than in that figured, owing to the greater length and 
slenderness often attained by the three divergent arms. The individual vessels of 
fig. 21 are now reduced to a very small size. In fig. 23 this reduction has gone further 
still, and the entire bundle is but a slender, cylindrical, vascular thread; whilst in 
Plate LIII. fig. 24 the whole stem is reduced to a small cylindrical body, looking more 
like the ultimate fibre of a root than a stem, and with two or three very small vessels 
in its centre, scarcely discernible amidst the cellular tissues that invest them. These 
smallest and ultimate fibres are not more than - 006 in their entire diameter. 
Unlike the greater number of the non-Lepidodendroid stems of the Coal-measures, 
that under consideration branches freely, and we are able to trace the process in the 
various sections represented by Plate LIII. figs. 25 A & B and Plate LIV. 
figs. 26 & 27. Fig. 25 A exhibits at a! some of the small peripheral vascular tissues 
already seen in fig. 20, a! ; but on the upper side of the section the corresponding 
vessels have become detached as well as increased in numbers, forming the two trian- 
gular bundles (a", a"), the inner bark (g) still connecting them with the two uppermost 
of the primary bundles ( a , a). In fig. 25 B we have a similar condition of things, but in 
a somewhat earlier stage of progress. The diverging tissues ( a ") are here less completely 
detached from the central bundles (a, a), and they have not yet in this instance undergone 
the division into two separate bundles which invariably takes place before these tissues 
emerge from the bark. Plate LIV. fig. 26 exhibits a smaller stem in which these trifid 
bundles (a") are completely detached from the central one (a). In all the above three 
cases the transverse sections have been made at points intermediate between those at 
which the central bundle first separates into three and those at which the branches 
dichotomize externally. This will be made sufficiently intelligible on referring to 
Plate LIV. fig. 27, which is a similar branching stem intersected longitudinally. In 
the latter section the common vascular bundle (a, a) is but slightly divided at its lower 
part into two bundles by some cellular tissue (g). Superiorly these two bundles (a’, a!) 
diverge — that on the right hand proceeding into the branch of its own side, whilst that 
on the left, and which is a somewhat smaller bundle, proceeds to the left-hand smaller 
